Effects of noise overexposure on distortion product otoacoustic emissions: Auditory hazard from impulsive occupational exposures

The risk of noise-induced hearing loss (NIHL) at the workplace can be predicted according to the International Standard ISO 1999:1990. The standard is applicable to all types of noise and it is based on measurements of the total acoustic energy (LEX,8ℎ). Therefore, noises with equal energy are assumed to be equally hazardous for our hearing. Nevertheless, the standard allows adding a +5dB penalty to impulsive and tonal noises based on the presumption that they might pose a higher risk of hearing loss.

This PhD thesis investigates the effect of different occupational noise exposures on the auditory system and the need for penalization. A total of 16 normal-hearing human subjects were exposed under laboratory conditions to three noise stimuli with equal energy: (1) continuous broadband; (2) impulsive+continuous; and (3) tonal. Temporary changes on the hearing of the subjects were evaluated with measurements of distortion product otoacoustic emissions (DPOAE). The assumption is that a difference in the temporary changes from the stimuli may be indicative of a higher risk of NIHL. The results suggest that the risk of NIHL for low-level impulses may be predicted on an energy basis, whereas the penalty may be more appropriate for tonal exposures. This presentation will also address methodological aspects of DPOAE measurements that might improve hearing diagnosis and detection of hearing loss.